The present invention relates to techniques for communicating primary and secondary data streams over a communication channel.
It is often desirable in data communications applications to transmit not only principal, or primary, data, but also secondary data. For example, present-day telephone voiceband modems--particularly those designed for leased-line operation--are often arranged to not only transmit the so-called user (primary) data, but also diagnostic and/or maintenance (secondary) data via which the modems themselves communicate.
For modems operating at relatively low data transmission rates, the available transmission bandwidth can be divided into so-called primary and secondary channels in which the primary and secondary data are respectively transmitted. At higher data rates, however, the primary channel bandwidth requirements may be such as to preclude this approach because every Hertz of bandwidth is needed for the primary channel.
As a consequence of the foregoing, various types of so-called inband secondary data signalling (hereinafter "inband signallin") schemes have been proposed in which the secondary data is transmitted along with the primary data in the primary channel. In accordance with a particular type of inband signalling scheme, an alphabet, or constellation, of "standard" signal points--having just enough signal points to accommodate the various possible primary data word values--is expanded to include a number of additional, or "expanded," signal points. Particular data word values to be communicated over the primary channel are representable by either of a predetermined pair of signal points associated with that value--one signal point of the pair being a standard signal point--referred to herein as a "secondary data standard signal point"--and the other being an expanded signal point. The specific one of the two signal points of the pair that is transmitted at any given time is chosen as a function of whether the value of the next bit in the secondary data stream waiting to be transmitted is a "0" or a "1". Schemes of this type are typified by the arrangements shown in U.S. Pat. No. 4,651,320 issued Mar. 17, 1987 to H. K. Thapar and U.S. Pat. No. 4,627,077 issued Dec. 2, 1986 to T. R. Armstrong.